Nonaqueous electrolyte, electrical double-layer capacitors, and nonaqueous electrolyte secondary cells

ABSTRACT

Secondary cells and electrical double-layer capacitors of excellent charge-discharge efficiency, stability and low-temperature properties can be obtained using nonaqueous electrolytes which contain an ionic liquid that has general formula (1) below and is liquid at not higher than 50° C. and an ion-conductive polymer. 
     
       
         
         
             
             
         
       
     
     In formula (1), R 1  to R 4  are each independently an alkyl group of 1 to 5 carbons or an alkoxyalkyl group of the formula R′—O—(CH 2 ) n — (R′ being methyl or ethyl, and the letter n being an integer from 1 to 4), and any two from among R 1 , R 2 , R 3  and R 4  may together form a ring, with the proviso that at least one of R 1  to R 4  is an alkoxyalkyl group of the above formula. X is a nitrogen atom or a phosphorus atom, and Y is a monovalent anion.

TECHNICAL FIELD

The present invention relates to nonaqueous electrolytes, and toelectrical double-layer capacitors and nonaqueous electrolyte secondarycells in which such nonaqueous electrolytes are used.

BACKGROUND ART

Ionic compounds generally exist in the form of crystals composed ofpositively charged cations and negatively charged anions which pullelectrostatically on each other. Such ionic compounds dissolve in waterand various other liquids to form liquids that conduct electricity,i.e., electrolyte solutions.

Some ionic compounds maintain a liquid state at room temperature and donot solidify even at very low temperatures. Such ionic compounds whichmaintain a liquid state at room temperature or below are referred to inparticular as “room-temperature fused salts” or “ionic liquids.” Tominimize electrostatic interactions between the cations and anions whichmake up the ionic liquid, either the cations or anions or both aremolecular ions of a significant size. Moreover, to minimize the chargeand electrostatic interactions, either or both are monovalent.

Active research efforts are being made to employ such ionic liquids aselectrolytes in batteries and other applications. However, in general,ionic liquids have a high hydroscopic property and are difficult tohandle in air. As a result, their use has remained limited.

In light of these circumstances, the 1-ethyl-3-methylimidazoliumtetrafluoroborate reported by Wilkes et al. in 1992 is a remarkableionic liquid that can be handled even in air. This new ionic liquid ledto the synthesis of many ionic liquids which are combinations ofnumerous alkylimidazolium cations having different side chains withvarious anions.

Such developments have gradually led to efforts to use ionic liquids aselectrolytes in nonaqueous electrolyte secondary cells. For example,JP-A 8-245828 (Patent Reference 1), JP-A 10-265673 (Patent Reference 2)and JP-A 10-265674 (Patent Reference 3) disclose solid electrolyteswhich use polymeric compound complexes containing a room-temperaturefused salt (ionic liquid) and a polymeric compound (and lithium salt).Solid electrolytes which employ such polymeric compound complexes areable to reduce the tendency for leakage associated with the use ofliquid electrolytes.

Each of the foregoing patent references mentions the use of substancessuch as cyclic amidine onium salts, pyridine onium salts and aliphaticquaternary ammonium salts of organic carboxylic acids asroom-temperature fused salts. However, because these room-temperaturefused salts do not have very broad potential windows, they are readilysubject to reductive decomposition during charging and discharging ofthe electrochemical devices such as secondary cells and tend to degrade,thus lacking a performance adequate for practical use.

Also, these room-temperature fused salts have relatively highsolidification points, and thus remain inadequate in terms of increasingthe low-temperature properties of electrochemical devices such assecondary cells.

Moreover, in the above-described electrolytes which employ a polymericcompound complex, because the polymer itself either lacks ionicconductivity or has only a poor ionic conductivity, the polymer complexobtained with the polymer has a greatly diminished ionic conductivity.

In light of these circumstances, one object of the present invention isto provide nonaqueous electrolytes which contain an ionic liquid and apolymeric compound and afford electrical double-layer capacitors andnonaqueous electrolyte secondary cells having excellent charge-dischargeefficiency, stability and low-temperature characteristics. Additionalobjects of the invention are to provide electrical double-layercapacitors and nonaqueous electrolyte secondary cells in which suchnonaqueous electrolytes are used.

DISCLOSURE OF THE INVENTION

The inventors have conducted extensive investigations in order toachieve the above objects. As a result, they have found that quaternaryammonium salts and quaternary phosphonium salts bearing at least onealkoxyalkyl group as a substituent have the properties of ionic liquids.Moreover, the inventors have found that because these ionic liquidsexhibit a liquid state even at low temperatures and have a broadpotential window, they are not readily subject to reductivedecomposition during the charging and discharging of a battery, forexample. In addition, the inventors have learned that by usingnonaqueous electrolytes containing such an ionic liquid and anion-conductive polymeric compound as the nonaqueous electrolyte inelectrical double-layer capacitors and nonaqueous electrolyte secondarycells, there can be obtained electrical double-layer capacitors andnonaqueous electrolyte secondary cells endowed with excellentcharge-discharge efficiency and excellent stability and low-temperaturecharacteristics. These discoveries led ultimately to the presentinvention.

Accordingly, the invention provides the following:

-   (1) A nonaqueous electrolyte characterized by containing an ionic    liquid which has general formula (1) below and is liquid at not    higher than 50° C.

-    wherein R¹ to R⁴ are each independently an alkyl group of 1 to 5    carbons or an alkoxyalkyl group of the formula R′—O—(CH₂)_(n)— (R′    being methyl or ethyl, and the letter n being an integer from 1 to    4), and any two from among R¹, R², R3 and R⁴ may together form a    ring, with the proviso that at least one of R¹ to R⁴ is an    alkoxyalkyl group of the above formula, X is a nitrogen atom or a    phosphorus atom, and Y is a monovalent anion; and containing also an    ion-conductive polymer.-   (2) A nonaqueous electrolyte which is characterized in that it is    obtained by curing a composition containing an ionic liquid which    has general formula (1) below and is liquid at not higher than 50°    C.

wherein R¹ to R⁴ are each independently an alkyl group of 1 to 5 carbonsor an alkoxyalkyl group of the formula R′—O—(CH₂)_(n)— (R′ being methylor ethyl, and the letter n being an integer from 1 to 4), and any twofrom among R¹, R², R³ and R⁴ may together form a ring, with the provisothat at least one of R¹ to R⁴ is an alkoxyalkyl group of the aboveformula, X is a nitrogen atom or a phosphorus atom, and Y is amonovalent anion; and containing also a compound having a reactivedouble bond on the molecule, and an ion-conductive polymer.

-   (3) The nonaqueous electrolyte of (1) or (2) above which is    characterized by containing a lithium salt.-   (4) The nonaqueous electrolyte of (3) above which is characterized    in that the lithium salt is LiBF₄, LiPF₆, Li(CF₃SO₂)₂N, LiCF₃SO₃ or    LiCF₃CO₂.-   (5) The nonaqueous electrolyte of any one of (1) to (4) above which    is characterized in that the ion-conductive polymer is a    noncrystalline polymer.-   (6) The nonaqueous electrolyte of any one of (1) to (5) above which    is characterized in that the ion-conductive polymer has a relative    permittivity at 25° C. and 1 MHz of 5 to 50.-   (7) The nonaqueous electrolyte of any one of (1) to (6) above which    is characterized in that the ion-conductive polymer is a    thermoplastic polyurethane resin.-   (8) The nonaqueous electrolyte of any one of (1) to (6) above which    is characterized in that the ion-conductive polymer is a    hydroxyalkyl polysaccharide or a hydroxyalkyl polysaccharide    derivative.-   (9) The nonaqueous electrolyte of any one of (1) to (6) above which    is characterized in that the ion-conductive polymer is a polymeric    compound having an average degree of polymerization of at least 20    and containing polyvinyl alcohol units of general formula (2) below

-    wherein n is a number from 20 to 10,000, some or all of the    hydroxyl groups on the polyvinyl alcohol units being substituted    with oxyalkylene-bearing groups having an average molar substitution    of at least 0.3.-   (10) The nonaqueous electrolyte of any one of (1) to (6) above which    is characterized in that the ion-conductive polymer is a polymeric    compound having an average degree of polymerization of at least 20    and containing polyvinyl alcohol units of general formula (2) below

-    wherein n is a number from 20 to 10,000, some or all of the    hydroxyl groups on the polyvinyl alcohol units being substituted    with cyano-substituted monovalent hydrocarbon groups.-   (11) The nonaqueous electrolyte of any one of (1) to (6) above which    is characterized in that the ion-conductive polymer is a polymeric    compound having units of formula (3) and units of formula (4)

-    wherein at least 10% of the end groups on the molecular chain are    capped with one or more groups selected from among halogen atoms,    substituted or unsubstituted monovalent hydrocarbon groups, R⁵CO—    groups (R⁵ being a substituted or unsubstituted monovalent    hydrocarbon group), R⁵ ₃Si— groups (R⁵ being the same as above),    amino groups, alkylamino groups, H(OR⁶)_(m)— groups (R⁶ being an    alkylene group of 2 to 4 carbons, and m being an integer from 1    to 100) and phosphorus atom-containing groups.-   (12) The nonaqueous electrolyte of any one of (1) to (11) above    which is characterized in that the ionic liquid is liquid at not    higher than 25° C.-   (13) The nonaqueous electrolyte of any one of (1) to (12) above    which is characterized in that X is a nitrogen atom, R′ is methyl,    and n is 2.-   (14) The nonaqueous electrolyte of any one of (1) to (12) above    which is characterized in that the ionic liquid has general    formula (5) below

-    wherein R′ is methyl or ethyl, X is a nitrogen atom or a phosphorus    atom, Y is a monovalent anion, Me stands for methyl and Et stands    for ethyl.-   (15) The nonaqueous electrolyte of any one of (1) to (14) above    which is characterized in that Y is BF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻,    CF₃SO₃ ⁻ or CF₃CO₂ ⁻.-   (16) An electrical double-layer capacitor comprising a pair of    polarizable electrodes, a separator between the polarizable    electrodes and a nonaqueous electrolyte; which electrical    double-layer capacitor is characterized in that the nonaqueous    electrolyte is a nonaqueous electrolyte according to any one of (1)    to (15) above.-   (17) A nonaqueous electrolyte secondary cell comprising a positive    electrode which contains a lithium-containing double oxide, a    negative electrode which contains a carbonaceous material capable of    lithium ions insertion and extraction or contains metallic lithium,    a separator between the positive and negative electrodes, and a    nonaqueous electrolyte; which nonaqueous secondary cell is    characterized in that the nonaqueous electrolyte is a nonaqueous    electrolyte according to any one of (1) to (15) above.

Because the nonaqueous electrolyte of the invention contains an ionicliquid that exhibits a liquid state even at low temperatures and has abroad potential window and contains also an ion-conductive polymer, ithas an excellent ionic conductivity, stability and other properties. Byusing this nonaqueous electrolyte as the electrolyte in secondary cellsand electrical double-layer capacitors, there can be obtained secondarycells and capacitors having an excellent charge-discharge efficiency andhaving excellent stability, cycle retention and low-temperaturecharacteristics.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described more fully below.

[Nonaqueous Electrolyte]

The ionic liquid used in the nonaqueous electrolyte according to theinvention has general formula (1) below and is in a liquid state at nothigher than 50° C.

In formula (1), R¹ to R⁴ are each independently an alkyl group of 1 to 5carbons or an alkoxyalkyl group of the formula R¹—O—(CH₂)_(n)— (R′ beingmethyl or ethyl, and the letter n being an integer from 1 to 4), and anytwo from among R¹, R², R³ and R⁴ may together form a ring, with theproviso that at least one of R¹ to R⁴ is an alkoxyalkyl group of theabove formula. X is a nitrogen atom or a phosphorus atom, and Y is amonovalent anion.

Examples of alkyls having 1 to 5 carbons include methyl, ethyl, propyl,2-propyl, butyl and pentyl. However, taking into account the physicalproperties and the electrochemical characteristics of the ionic liquid,it is preferable for at least one of groups R¹ to R⁴ to be methyl, ethylor propyl, and especially methyl or ethyl. These ethyl or propyl groupsmay form rings with other alkyl groups.

Examples of alkoxyalkyl groups of the formula R′—O—(CH₂)_(n)— includemethoxymethyl, ethoxymethyl, methoxyethyl, ethoxyethyl, methoxypropyl,ethoxypropyl, methoxybutyl and ethoxybutyl. The letter n is an integerfrom 1 to 4. However, taking into account the physical properties andthe electrochemical characteristics of the ionic liquid, the letter n ispreferably 1 or 2, and most preferably 2.

Exemplary compounds in which any two groups from among R¹ to R⁴ form aring include, when X is a nitrogen atom, quaternary ammonium saltscontaining an aziridine, azetidine, pyrrolidine or piperidine ring; and,when X is a phosphorus atom, quaternary phosphonium salts containing apentamethylenephosphine (phosphorinane) ring.

Quaternary ammonium salts having as a substituent at least onemethoxyethyl group, in which R′ above is methyl and the letter n is 2,are preferred.

Preferred use can also be made of quaternary salts of general formula(5) below having as substituents a methyl group, two ethyl groups and analkoxyethyl group, and compounds of general formula (6) below having assubstituents two methyl groups, an ethyl group and an alkoxyethyl group.

In formulas (5) and (6), R′ is methyl or ethyl, X is a nitrogen orphosphorus atom, and Y is a monovalent anion. Me represents a methylgroup and Et represents an ethyl group.

Illustrative, non-limiting examples of the monovalent anion Y includeBF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, AlCl₄ ⁻, HSO₄ ⁻, ClO₄ ⁻, CH₃SO₃ ⁻, CF₃SO₃⁻, CF₃CO₂ ⁻, (CF₃SO₂)₂N⁻, Cl⁻, BR⁻ and I⁻. To ensure such properties asa good degree of dissociation and good stability, the use of BF₄ ⁻, PF₆⁻, (CF₃SO₂)₂N⁻, CF₃SO₃ ⁻ or CF₃CO₂ ⁻ is preferred.

Of these anions, the use of (CF₃SO₂)₂N⁻ is highly preferable for furtherreducing the viscosity of the ionic liquid and increasing itshandleability. BF₄ ⁻ is also highly preferable because the resultingionic liquid has a high versatility and is less readily affected bywater than ionic liquids containing PF₆ ⁻ as the anion and thus easierto handle.

Specific examples of ionic liquids highly suitable for use in theinvention include compounds (7) to (19) below (wherein “Me” stands formethyl and “Et” stands for ethyl).

A common method for synthesizing such quaternary ammonium salts isdescribed. First, a tertiary amine is mixed with a compound such as analkyl halide or a dialkyl sulfate and reacted under heating, ifnecessary, to give a quaternary ammonium halide. In cases where acompound having a low reactivity (e.g., an alkoxyethyl halide or analkoxymethyl halide) is used, reaction under applied pressure, such asin an autoclave, is desirable.

The resulting quaternary ammonium halide is dissolved in an aqueoussolvent such as water, then reacted with a reagent that generates therequired anionic species, such as tetrafluoroboric acid ortetrafluorophosphoric acid, so as to effect an anion exchange reaction,yielding the quaternary ammonium salt.

In one illustrative method for synthesizing quaternary ammoniumtetrafluoroborate, a quaternary ammonium halide is dissolved in water,silver oxide is added and a salt exchange reaction is carried out toform the corresponding quaternary ammonium hydroxide. The product isthen reacted with tetrafluoroboric acid, yielding the target compound.This method is effective for synthesizing high-purity quaternaryammonium tetrafluoroborates because the silver halide that arises as aresult of salt exchange during formation of the quaternary ammoniumhydroxide can easily be removed.

Quaternary phosphonium salts can generally be synthesized in much thesame way as quaternary ammonium salts. Typically, a tertiary phosphineis mixed with a suitable compound such as an alkyl halide or a dialkylsulfate. If necessary, the reaction is carried out under the applicationof heat.

As in the case of quaternary ammonium salts, quaternary phosphoniumsalts containing various different anions may be prepared by dissolvinga quaternary phosphonium halide (a chloride, bromide or iodide) in anaqueous solvent and reacting the dissolved halide with a reagent thatgenerates the required anionic species so as to effect an anion exchangereaction.

The above ionic liquid is in a liquid state at a temperature not higherthan 50° C., preferably not higher than 25° C., and most preferably nothigher than 15° C. Because nonaqueous electrolyte secondary cells andelectrical double-layer capacitors are normally used at temperatures offrom 50° C. down to −10° C., there is no point in using an ionic liquidwhich is not in a liquid state within this temperature range. The lowerthe temperature at which the ionic liquid is in a liquid state, the moredesirable because this broadens the temperature range in which thenonaqueous electrolyte secondary cell and the electrical double-layercapacitor can be used.

Because the ionic liquid of the invention is in a liquid state at alower temperature than the imidazolium ion-containing ionic liquidswhich have hitherto been used, by employing a nonaqueous electrolytecontaining this ionic liquid as the electrolyte in a nonaqueouselectrolyte secondary cell or an electrical double-layer capacitor,secondary cells and electrical double-layer capacitors having evenbetter low-temperature characteristics can be obtained.

Also, because the ionic liquid has a broad potential window and is notitself readily subject to reductive decomposition during charging anddischarging, there can be obtained an electrolyte which resistsdeterioration even when charging and discharging are repeatedly carriedout. As a result, highly stable secondary cells and electricaldouble-layer capacitors can be achieved.

The first nonaqueous electrolyte according to the invention contains theabove-described ionic liquid and an ion-conductive polymer.

The ion-conductive polymer here is not subject to any particularlimitation, although to manifest a high ionic conductivity it ispreferable for this polymer to be non-crystalline.

Also, in general, cation and anion dissociation is strongly promotedwithin a highly polar matrix . Accordingly, the ionic conductivity canbe substantially increased by a method involving mixture with a highlypolar polymer. From this standpoint, it is preferable to use as theion-conductive polymer a polymer having a relative permittivity at 25°C. and 1 MHz of from 5 to 50, and especially from 10 to 50. To increasethe polarity of the polymer matrix, it is desirable to introducesubstituents having a large dipole moment onto the polymer. Onepreferred example of such substituents is the cyano group.

Ion-conductive polymers having all the above characteristics that arepreferable for use include (a) hydroxyalkyl polysaccharide derivatives,(b) oxyalkylene branched polyvinyl alcohol derivatives, (c) polyglycidolderivatives, (d) cyano-substituted monovalent hydrocarbon group-bearingpolyvinyl alcohol derivatives, and (e) thermoplastic polyurethaneresins.

Illustrative examples of (a) hydroxyalkyl polysaccharide derivativesinclude: (1) hydroxyethyl polysaccharides prepared by reacting ethyleneoxide with a naturally occurring polysaccharide such as cellulose,starch or pullulan; (2) hydroxypropyl polysaccharides prepared byreacting propylene oxide with the above naturally occurringpolysaccharides; and (3) dihydroxypropyl polysaccharides prepared byreacting glycidol or 3-chloro-1,2-propanediol with the above naturallyoccurring polysaccharides. Hydroxyalkyl polysaccharide derivatives inwhich some or all of the hydroxyl groups on these hydroxyalkylpolysaccharides are capped with an ester-bonded or ether-bondedsubstituent are preferred.

The above hydroxyalkyl polysaccharides have a molar substitution of 2 to30, and preferably 2 to 20. At the molar substitution of less than 2,the salt dissolving ability of the hydroxyalkyl polysaccharide maybecome so low as to make it unsuitable for use.

Oxyalkylene branched polyvinyl alcohol derivatives (b) suitable for useas the polymeric compound include polymeric compounds which bear on themolecule polyvinyl alcohol units of general formula (2) below, whichhave an average degree of polymerization of at least 20, and in whichsome or all of the hydroxyl groups on the polyvinyl alcohol units aresubstituted with oxyalkylene-bearing groups having an average molarsubstitution of at least 0.3.

In formula (2), the letter n is preferably from 20 to 10,000.

Because this type of polymeric compound has a high oxyalkylene fraction,it has the ability to dissolve a large amount of salt. In addition, themolecule contains many oxyalkylene segments which permit the movement ofions, resulting in a high ion mobility. This type of polymeric compoundis thus capable of exhibiting a high ionic conductivity. Moreover, thesepolymeric compounds have a high tackiness. Accordingly, they act as abinder component and are capable of firmly bonding the positive andnegative electrodes.

Examples of polymeric compounds of above formula (2) include [1]polymeric compounds obtained by reacting a polyvinyl alcoholunit-containing polymeric compound with an oxirane compound such asethylene oxide, propylene oxide or glycidol (e.g., dihydroxypropylatedpolyethylene vinyl alcohol, propylene oxide-modified polyvinyl alcohol);and [2] polymeric compounds obtained by reacting a polymeric compoundhaving polyvinyl alcohol units with a polyoxyalkylene compound havingterminal hydroxy-reactive substituents.

Here, the polyvinyl alcohol unit-bearing polymeric compound is apolymeric compound which has polyvinyl alcohol units on the molecule,which has a number-average degree of polymerization of at least 20,preferably at least 30, and most preferably at least 50, and in whichsome or all of the hydroxyl groups on the polyvinyl alcohol units aresubstituted with oxyalkylene-bearing groups. For the sake ofhandleability, the upper limit in the number-average degree ofpolymerization in this case is preferably not more than 2,000, morepreferably not more than 500, and most preferably not more than 200.

It is most preferable for the above-described polyvinyl alcoholunit-bearing polymeric compound to have a number-average degree ofpolymerization within the above range and to be a homopolymer in whichthe fraction of polyvinyl alcohol units in the molecule is at least 98mol %. However, the polyvinyl alcohol unit-bearing polymeric compound isnot limited to the above, and may be one which has a number-averagedegree of polymerization within the above range and which has apolyvinyl alcohol fraction of preferably at least 60 mol %, and morepreferably at least 70 mol %. Illustrative examples of such compoundsthat may be used include polyvinyl formals in which some of the hydroxylgroups on the polyvinyl alcohol have been converted to formal, modifiedpolyvinyl alcohols in which some of the hydroxyl groups on the polyvinylalcohol have been converted to alkyls, poly(ethylene vinyl alcohols),partially saponified polyvinyl acetates, and other modified polyvinylalcohols.

This polymeric compound is one in which some or all of the hydroxylgroups on the above-described polyvinyl alcohol units are substitutedwith oxyalkylene-containing groups having an average molar substitutionof at least 0.3 (moreover, some of the hydrogen atoms on theseoxyalkylene groups may be substituted with hydroxyl groups). Preferablyat least 30 mol %, and most preferably at least 50 mol %, of thehydroxyl groups are substituted in this way.

The above-mentioned polyglycidol derivative (c) contains units offormula (3) (referred to hereinafter as “A units”)

and units of formula (4) (referred to hereinafter as “B units”)

The ends of the molecular chain are capped with specific substituents.

The polyglycidol can be prepared by polymerizing glycidol or3-chloro-1,2-propanediol, although it is generally preferable to carryout polymerization from glycidol as the starting material and using abasic catalyst or a Lewis acid catalyst.

The total number of A and B units on the polyglycidol molecule is atleast two, preferably at least six, and most preferably at least ten.There is no particular upper limit, although it is generally preferablefor the total number of such units to not exceed about 10,000. The totalnumber of these respective units may be set as appropriate based on suchconsiderations as the flow ability and viscosity required of thepolyglycidol. The ratio of A units to B units in the molecule, expressedas A/B, is within a range of 1/9 to 9/1, and preferably 3/7 to 7/3.There is no regularity to the arrangement of A and B units; anycombination is possible

The polyglycidol has a polyethylene glycol equivalent weight-averagemolecular weight (Mw), as determined by gel permeation chromatography(GPC), within a range of preferably 200 to 730,000, more preferably 200to 100,000, and most preferably 600 to 20,000. The dispersity, definedas weight-average molecular weight divided by number-average molecularweight (Mw/Mn) is preferably 1.1 to 20, and most preferably 1.1 to 10.

These polymeric compounds (a) to (c) may be hydroxyl-capped polymerderivatives in which some or all, and preferably at least 10 mol %, ofthe hydroxyl groups on the molecule are capped with one or more type ofmonovalent substituent selected from among halogen atoms, substituted orunsubstituted monovalent hydrocarbon groups having 1 to 10 carbons,R⁵CO— groups (wherein R⁵ is a substituted or unsubstituted monovalenthydrocarbon group of 1 to 10 carbons), R⁵ ₃Si— groups (wherein R⁵ is asdefined above), amino groups, alkylamino groups and phosphorusatom-containing groups.

Illustrative examples of the substituted or unsubstituted monovalenthydrocarbon groups having 1 to 10 carbons include alkyl groups such asmethyl, ethyl, propyl, isopropyl, t-butyl and pentyl, aryl groups suchas phenyl and tolyl, aralkyl groups such as benzyl, alkenyl groups suchas vinyl, and any of the foregoing in which some or all of the hydrogenatoms have been substituted with halogen atoms, cyano groups, hydroxylgroups or amino groups. Any one or combination of two or more of thesetypes of groups may be used.

Capping the hydroxyl groups on the above polymeric compounds (a) to (c)with highly polar substituents increases the polarity (and thus therelative permittivity) of the polymer matrix, making it possible toprevent the decline in conductivity which readily arises in a lowrelative permittivity polymer matrix due to the recombination ofdissociated cations and counteranions. Moreover, when capping is doneusing substituents that have fire-retarding and hydrophobic properties,the polymeric compound can be imparted with desirable characteristicssuch as hydrophobicity and fire retardance.

To increase the relative permittivity of above polymeric compounds (a)to (c), the oxyalkylene chain-bearing polymeric compounds (a) to (c) arereacted with a hydroxy-reactive compound so as to cap the hydroxylgroups on these polymeric compounds with highly polar substituents.

Although the highly polar substituents used for this purpose are notsubject to any particular limitation, neutral substituents arepreferable to ionic substituents. Exemplary substituents includesubstituted and unsubstituted monovalent hydrocarbon groups of 1 to 10carbons, and R⁵CO— groups (wherein R⁵ is as defined above). Ifnecessary, capping may also be carried out with other suitablesubstituents, such as amino groups or alkylamino groups.

To confer polymeric compounds (a) to (c) with hydrophobic properties andfire retardance, the hydroxyl groups on the above polymeric compoundsmay be capped with, for example, halogen atoms, R⁵ ₃Si— groups (whereinR⁵ is as defined above) or phosphorus-containing groups.

Examples of suitable R⁵ ₃Si— groups include those in which R⁵ representsthe same substituted or unsubstituted monovalent hydrocarbon groupshaving 1 to 10 carbons, and preferably 1 to 6 carbons, as above. R⁵preferably stands for alkyl groups. Trialkylsilyl groups, and especiallytrimethylsilyl groups, are preferred.

Additional examples of suitable substituents include amino groups,alkylamino groups and phosphorus atom-containing groups.

The proportion of end groups capped with the above substituents is atleast 10 mol %, preferably at least 50 mol %, and most preferably atleast 90 mol %. It is even possible to cap substantially all the endgroups with the above substituents, representing a capping ratio ofabout 100 mol %.

The above-mentioned cyano-substituted monovalent hydrocarbongroup-bearing polyvinyl alcohol derivative (d) is preferably a polymericcompound which bears on the molecule polyvinyl alcohol units of abovegeneral formula (2), which has an average degree of polymerization of atleast 20, and in which some or all of the hydroxyl groups on thepolyvinyl alcohol units are substituted with cyano-substitutedmonovalent hydrocarbon groups.

Because this polymeric compound has relatively short side chains, theviscosity of the electrolyte can be held to a low level.

Examples of such polymeric compounds include polyvinyl alcohols in whichsome or all of the hydroxyl groups have been substituted with groupssuch as cyanoethyl, cyanobenzyl or cyanobenzoyl. Cyanoethyl-substitutedpolyvinyl alcohols are especially preferred because the side chains areshort.

Various known methods may be used to substitute the hydroxyl groups onthe polyvinyl alcohol with cyano-substituted monovalent hydrocarbongroups.

The above-mentioned thermoplastic polyurethane resin (e) is preferably athermoplastic polyurethane resin prepared by reacting (A) a polyolcompound, (B) a polyisocyanate compound and, if necessary, (C) a chainextender.

Suitable thermoplastic polyurethane resins include not only polyurethaneresins having urethane bond, but also polyurethane-urea resins havingboth urethane bond and urea bond.

The polyol compound serving as component A is preferably a polyetherpolyol, a polyester polyol, a polyester polyether polyol, a polyesterpolycarbonate polyol, a polycaprolactone polyol, or a mixture thereof.

The polyol compound serving as component A has a number-averagemolecular weight of preferably 1,000 to 5,000, and most preferably 1,500to 3,000. A polyol compound having too small a number-average molecularweight may lower the physical properties of the resulting thermoplasticpolyurethane resin film, such as the heat resistance and tensileelongation percentage. On the other hand, too large a number-averagemolecular weight increases the viscosity during synthesis, which maylower the production stability of the thermoplastic polyurethane resinbeing prepared. The number-average molecular weights used here inconnection with polyol compounds are calculated based on the hydroxylvalues measured in accordance with JIS K1577.

Illustrative examples of the polyisocyanate compound serving ascomponent B include aromatic diisocyanates such as tolylenediisocyanate, 4,4′-diphenylmethane diisocyanate, p-phenylenediisocyanate, 1,5-naphthylene diisocyanate and xylylene diisocyanate;and aliphatic or alicyclic diisocyanates such as hexamethylenediisocyanate, isophorone diisocyanate, 4,4′-dicyclohexylmethanediisocyanate and hydrogenated xylylene diisocyanate.

The chain extender serving as component C is preferably alow-molecular-weight compound having a molecular weight of not more than300 and bearing two active hydrogen atoms capable of reacting withisocyanate groups.

Various known compounds may be used as such low-molecular-weightcompounds. Illustrative examples include aliphatic diols such asethylene glycol, propylene glycol and 1,3-propanediol; aromatic oralicyclic diols such as 1,4-bis (β-hydroxyethoxy)benzene,1,4-cyclohexanediol and bis (βhydroxyethyl) terephthalate; diamines suchas hydrazine, ethylenediamine, hexamethylenediamine and xylylenediamine;and amino alcohols such as adipoyl hydrazide. Any one or combinations oftwo or more of these may be used.

The thermoplastic polyurethane resin typically includes 5 to 200 partsby weight, and preferably 20 to 100 parts by weight, of thepolyisocyanate compound serving as component B and 1 to 200 parts byweight, and preferably 5 to 100 parts by weight, of the chain extenderserving as component C per 100 parts by weight of the polyol compoundserving as component A.

A lithium salt can also be added to the first nonaqueous electrolytedescribed above. The lithium salt employed for this purpose may be anyknown lithium salt capable of being used in nonaqueous electrolytesecondary cells. To ensure such properties as versatility and both goodsolubility and a high degree of dissociation in the ionic liquid, theuse of LiBF₄, LiPF₆, Li(CF₃SO₂)₂N, LiCF₃SO₃ or LiCF₃CO₂ is especiallypreferred.

The content of lithium salt in the above electrolyte is not subject toany particular limitation, although the content is generally 0.05 to 3mol/L, and preferably 0.1 to 2 mol/L. Too low a lithium saltconcentration may result in a higher cell impedance, which make chargingand discharging at a large current impossible. On the other hand, alithium salt concentration which is too high increases the liquidviscosity, which may make battery and capacitor production difficult.

In addition, if necessary, the above-described electrolyte may haveadded thereto cyclic or acylic esters, cyclic carboxylates, cyclic oracyclic ethers, phosphates, lactone compounds, nitrile compounds, amidecompounds, and mixtures thereof.

Exemplary cyclic carbonates include alkylene carbonates such aspropylene carbonate (PC), ethylene carbonate (EC) and butylenecarbonate; and vinylene carbonate (VC). Exemplary acyclic carbonatesinclude dialkyl carbonates such as dimethyl carbonate (DMC), methylethyl carbonate (MEC) and diethyl carbonate (DEC). Exemplary acycliccarboxylates include methyl acetate and methyl propionate. Exemplarycyclic and acyclic ethers include tetrahydrofuran, 1,3-dioxolane and1,2-dimethoxyethane. Exemplary phosphates include trimethyl phosphate,triethyl phosphate, ethyldimethyl phosphate, diethylmethyl phosphate,tripropyl phosphate, tributyl phosphate, tri(trifluoromethyl) phosphate,tri(trichloromethyl) phosphate, tri(trifluoroethyl) phosphate,tri(triperfluoroethyl) phosphate, 2-ethoxy-1,3,2-dioxaphosphoran-2-one,2-trifluoroethoxy-1,3,2-dioxaphosphoran-2-one and2-methoxyethoxy-1,3,2-dioxaphosphoran-2-one. An example of a suitablelactone compound is γ-butyrolactone. An example of a suitable nitrilecompound is acetonitrile. An example of a suitable amide compound isdimethylformamide. Of these compounds, cyclic carbonates, phosphates,and mixtures thereof are preferred.

As explained above, the nonaqueous electrolyte of the invention, becauseit includes a specific ionic liquid, can provide nonaqueous electrolytesecondary cells and electrical double-layer capacitors which undergolittle deterioration of cyclability and thus have an excellentstability, and which have excellent low-temperature characteristics.

Moreover, because this nonaqueous electrolyte has a broader potentialwindow than ionic liquids known to the prior art, the ionic liquiditself does not readily undergo reductive decomposition during chargingand discharging. As a result, the cycle retention and stability ofelectrochemical devices such as nonaqueous electrolyte secondary cellswhich use this electrolyte can be improved. Also, because the aboveionic liquid exhibits a liquid state at lower temperatures thanprior-art ionic liquids, nonaqueous electrolytes having betterlow-temperature characteristics can be obtained.

Furthermore, because the above nonaqueous electrolyte includes theabove-described conductive polymeric compound, it can manifest a highionic conductivity, in addition to which it can act as a bindercomponent and is fully capable of firmly bonding together the positiveand negative electrodes.

The second nonaqueous electrolyte of the invention is obtained by curinga composition containing the above-described ionic liquid andion-conductive polymer, and containing also a compound having a reactivedouble bond on the molecule. In this invention, curing is a conceptwhich encompasses also gelation.

That is, in cases where the nonaqueous electrolyte obtained by curing orgelating the above composition is formed into a thin film and used asthe electrolyte in a secondary cell or capacitor, to increase thephysical strength (e.g., shape retention), a compound having a reactivedouble bond on the molecule and an ion-conductive polymer are added, andthe compound is reacted to form a polymer.

It is particularly desirable for the compound bearing a reactive doublebond on the molecule to have two or more reactive double bonds, becausethe reaction of such a compound forms a three-dimensional networkstructure, making it possible to increase even further the shaperetaining ability of the electrolyte.

When the nonaqueous electrolyte of the invention includes not only theabove-mentioned compound having at least two reactive double bonds, butalso the above-described conductive polymeric compound, there can beobtained an electrolyte having a semi-interpenetrating polymer network(semi-IPN) structure in which the molecular chains of the polymericcompound are intertwined with the three-dimensional network structure ofthe polymer formed by crosslinkage of the reactive double bond-bearingcompound. The shape retention and strength of the electrolyte can thusbe further increased, and its adhesive properties and ion conductivityalso enhanced.

The second nonaqueous electrolyte may also include within thecomposition the same lithium salt as that described above in connectionwith the first nonaqueous electrolyte. The amount of such lithium saltincluded may be set within the same range as described above for thefirst nonaqueous electrolyte.

Likewise, ion-conductive polymers that may be used in the secondnonaqueous electrolyte are of the same type as those described above inconnection with the first nonaqueous electrolyte. The amount ofion-conductive polymer included is not subject to any particularlimitation, although it is preferable for the weight ratio(ion-conductive polymer/reactive double bond-containing compound) to beset within a range of 0.001 to 0.1, and especially 0.003 to 0.005.

In addition, if necessary, the same cyclic or acyclic esters, acycliccarboxylates, cyclic or acyclic ethers, phosphates, lactone compounds,nitrile compounds, amide compounds or mixtures thereof as describedabove in connection with the first nonaqueous electrolyte may also beincluded.

The compound having a reactive double bond on the molecule is notsubject to any particular limitation. Illustrative examples includeacrylates and methacrylates such as glycidyl methacrylate, glycidylacrylate, methoxydiethylene glycol methacrylate, methoxytriethyleneglycol methacrylate and methoxypolyethylene glycol methacrylate (averagemolecular weight, 200 to 1,200); and other compounds having one acrylicacid group or methacrylic acid group on the molecule, such asmethacryloyl isocyanate, 2-hydroxymethylmethacrylic acid andN,N-dimethylaminoethylmethacrylic acid.

In cases where a semi-IPN structure is to be formed using such acompound having a single reactive double bond and the ion-conductivepolymeric compound described above, it is necessary to add also acompound having at least two reactive double bonds on the molecule.

Preferred examples of the compound having two or more reactive doublebonds on the molecule include divinylbenzene, divinylsulfone, allylmethacrylate, ethylene glycol dimethacrylate, diethylene glycoldimethacrylate, triethylene glycol dimethacrylate, polyethylene glycoldimethacrylate (average molecular weight, 200 to 1,000), 1,3-butyleneglycol dimethacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycoldimethacrylate, polypropylene glycol dimethacrylate (average molecularweight, 400), 2-hydroxy-1,3-dimethacryloxypropane, 2,2-bis[4-(methacryloxyethoxy)phenyl]propane, 2,2-bis[4-(methacryloxyethoxy-diethoxy)phenyl]propane, 2,2-bis[4-(methacryloxyethoxy-polyethoxy)phenyl]propane, ethylene glycoldiacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate,polyethylene glycol diacrylate (average molecular weight, 200 to 1,000),1,3-butylene glycol diacrylate, 1,6-hexanediol diacrylate, neopentylglycol diacrylate, polypropylene glycol diacrylate (average molecularweight, 400), 2-hydroxy-1,3-diacryloxypropane, 2,2-bis[4-(acryloxyethoxy)phenyl]propane, 2,2-bis[4-(acryloxyethoxy-diethoxy)phenyl]propane, 2,2-bis[4-(acryloxyethoxy-polyethoxy)phenyl]propane, trimethylolpropanetriacrylate, trimethylolpropane trimethacrylate, tetramethylolmethanetriacrylate, tetramethylolmethane tetraacrylate, water-soluble urethanediacrylate, water-soluble urethane dimethacrylate, tricyclodecanedimethanol acrylate, hydrogenated dicyclopentadiene diacrylate,polyester diacrylate and polyester dimethacrylate.

Of the aforementioned reactive double bond-bearing compounds, especiallypreferred reactive monomers include the polyoxyalkylenecomponent-bearing diesters of general formula (15) below. The use ofsuch a diester in combination with a polyoxyalkylene component-bearingmonoester of general formula (16) below and a triester is recommended.

In formula (15), R⁶ to R⁸ are each independently a hydrogen atom or analkyl group having 1 to 6 carbons, and preferably 1 to 4 carbons, suchas methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl ort-butyl; and X and Y satisfy the condition X≧1 and Y≧0 or the conditionX≧0 and Y≧1. R⁶ to R⁸ are preferably methyl, ethyl, n-propyl, i-propyl,n-butyl, i-butyl, s-butyl or t-butyl.

In formula (16), R⁹ to R¹¹ are each independently a hydrogen atom or analkyl group having 1 to 6 carbons, and preferably 1 to 4 carbons, suchas methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, s-butyl ort-butyl; and A and B satisfy the condition A≧1 and B≧0 or the conditionA≧0 and B≧2. R⁹ to R¹¹ are preferably methyl, ethyl, n-propyl, i-propyl,n-butyl, i-butyl, s-butyl or t-butyl.

A preferred example of the compound of above formula (15) is one inwhich X is 9, Y is 0, and both R⁶ and R⁸ are CH₃. A preferred example ofthe compound of above formula (16) is one in which A is 2 or 9, B is 0,and both R⁹ and R¹¹ are CH₃.

The triester is preferably trimethylolpropane trimethacrylate.

The above-described polyoxyalkylene component-bearing diester andpolyoxyalkylene component-bearing monoester are exposed, in a mixturetogether with the above-described ionic liquid and polymeric compound,to a suitable form of radiation (e.g., UV light, electron beams, x-rays,gamma rays, microwaves, radio-frequency radiation). Alternatively, themixture is heated to form a semi-IPN-type three-dimensional crosslinkednetwork structure.

The relative proportions of the above-described polyoxyalkylenecomponent-bearing diester, monoester and triester are set as appropriatefor the length of the polyoxyalkylene components and are not subject toany particular limitation. However, a diester/monoester molar ratio of0.1 to 2, and especially 0.3 to 1.5, and a diester/triester molar ratioof 2 to 15, and especially 3 to 10, are preferred to enhance thestrength of the electrolyte.

As explained above, the polymer electrolyte obtained by curing(gelating) a composition containing the nonaqueous electrolyte of theinvention and a reactive double bond-containing compound, in addition tohaving the above-mentioned properties such as low-temperaturecharacteristics, cyclability, ionic conductivity and tackiness, also hasa high shape-retaining ability.

In particular, because the electrolyte obtained by curing a compositionwhich includes a compound bearing at least two reactive double bonds asthe compound having a reactive double bond on the molecule and whichcontains also the above-described polymeric compound has a semi-IPN typethree-dimensional crosslinked network structure, the shape retainingability and strength of the electrolyte can be increased all the more,as can also its adhesive properties and ionic conductivity.

[Electrical Double-Layer Capacitor]

The electrical double-layer capacitor according to this invention is anelectrical double-layer capacitor having a pair of polarizableelectrodes, a separator between the polarizable electrodes, and theabove-described nonaqueous electrolyte.

The polarizable electrodes used here may be ones obtained by coating acurrent collector with a polarizable electrode composition containing acarbonaceous material and a binder polymer.

The carbonaceous material is not subject to any particular limitation.Illustrative examples include carbonaceous materials prepared by thecarbonization of a suitable starting material, or by both carbonizationand subsequent activation of the carbonized material to yield activatedcarbon. Examples of suitable starting materials include plant-basedmaterials such as wood, sawdust, coconut shells and pulp spent liquor;fossil fuel-based materials such as coal and petroleum fuel oil, as wellas fibers spun from coal or petroleum pitch obtained by the thermalcracking of such fossil fuel-based materials or from tar pitch; andsynthetic polymers, phenolic resins, furan resins, polyvinyl chlorideresins, polyvinylidene chloride resins, polyimide resins, polyamideresins, polycarbodiimide resins, liquid-crystal polymers, plastic wasteand reclaimed tire rubber.

The method of activation is not subject to any particular limitation.Any of various methods, such as chemical activation and steamactivation, may be used. However, activated carbons prepared by chemicalactivation using potassium hydroxide are especially preferred becausethey tend to provide a larger capacitance than steam-activated product.

The carbonaceous material used in the invention may be in any of variousforms, including shredded material, granulated material, pellets,fibers, felt, woven fabric or sheet.

A conductive material may be added to the carbonaceous material. Theconductive material may be any suitable material capable of conferringelectrical conductivity to the carbonaceous material. Illustrative,non-limiting, examples include carbon black, Ketjenblack, acetyleneblack, carbon whiskers, carbon fibers, natural graphite, artificialgraphite, titanium oxide, ruthenium oxide, and metallic fibers such asthose made of aluminum and nickel. Any one or combinations of two ormore thereof may be used. Of these, Ketjenblack and acetylene black,which are both types of carbon black, are preferred.

The average particle size of the conductive material, though not subjectto any particular limitation, is preferably 10 nm to 10 μm, morepreferably 10 to 100 nm, and even more preferably 20 to 40 nm. It isespecially advantageous for the conductive material to have an averageparticle size which is from 1/5000 to ½, and preferably from 1/1000 to1/10, the average particle size of the carbonaceous material.

The amount of conductive material added is not subject to any particularlimitation, although an amount of from 0.1 to 20 parts by weight, andpreferably 0.5 to 10 parts by weight, per 100 parts by weight of thecarbonaceous material is desirable for achieving a good electrostaticcapacitance and imparting electrical conductivity.

The binder polymer mentioned above may be any polymer capable of beingused in the applications of concern here. For example, use can be madeof various known binder polymers, such as polytetrafluoroethylene,polyvinylidene fluoride, carboxymethyl cellulose, fluoroolefincopolymer-type crosslinked polymers, polyvinyl alcohol, polyacrylicacid, polyimide, petroleum pitch, coal pitch and phenolic resins.

It is especially preferred to use as the binder polymer (I) athermoplastic resin having a swelling ratio, as defined by the formulabelow, in a range of 150 to 800%, (II) a fluoropolymer material, or acombination of two or more polymers of types (I) and (II).

The above thermoplastic resin (I) has a swelling ratio, as determinedfrom the formula indicated below, within a range of 150 to 800%,preferably 250 to 500%, and most preferably 250 to 400%.

${{Swelling}\mspace{14mu}{ratio}\mspace{14mu}(\%)} = {\frac{\begin{matrix}{{Weight}\mspace{14mu}{in}\mspace{14mu}{grams}\mspace{14mu}{of}\mspace{14mu}{swollen}\mspace{14mu}{thermoplastic}} \\{{resin}\mspace{14mu}{after}\mspace{14mu} 24\mspace{14mu}{hours}\mspace{14mu}{immersion}\mspace{14mu}{in}} \\{{electrolyte}\mspace{14mu}{solution}\mspace{14mu}{at}\mspace{14mu} 20{^\circ}\mspace{14mu}{C.\mspace{14mu}(g)}}\end{matrix}}{\begin{matrix}{{Weight}\mspace{14mu}{in}\mspace{14mu}{grams}\mspace{14mu}{of}\mspace{14mu}{thermoplastic}} \\{{resin}\mspace{14mu}{before}\mspace{14mu}{immersion}\mspace{14mu}{in}} \\{{electrolyte}\mspace{14mu}{solution}\mspace{14mu}{at}\mspace{14mu} 20{^\circ}\mspace{14mu}{C.\mspace{14mu}(g)}}\end{matrix}} \times 100}$

A thermoplastic resin containing units of general formula (17) below

wherein the letter r is 3 to 5 and the letter s is an integer ≧5, may beused as the binder polymer of formula (I) above.

Preferred examples of fluoropolymer materials (II) that may be used asthe binder polymer include polyvinylidene fluoride (PVDF), vinylidenefluoride-hexafluoropropylene copolymers (P(VDF-HFP)) and vinylidenefluoride-chlorotrifluoroethylene copolymers (P(VDF-CTFE)). Of these,fluoropolymers having a vinylidene fluoride content of at least 50 wt %,and especially at least 70 wt %, are preferred. The upper limit in thevinylidene fluoride content of the fluoropolymer is about 97 wt %.

The weight-average molecular weight of the fluoropolymer is not subjectto any particular limitation, although the weight-average molecularweight is preferably 500,000 to 2,000,000, and most preferably 500,000to 1,500,000. Too low a weight-average molecular weight may result in anexcessive decline in physical strength.

It is preferable for these binder polymers to be added in an amount of0.5 to 20 parts by weight, and especially 1 to 10 parts by weight, per100 parts by weight of the carbonaceous material.

No particular limitation is imposed on the method of preparing thepolarizable electrode composition. For example, the composition may beprepared by rendering the above-described carbonaceous material andbinder polymer into the form of a solution. If necessary, a solvent maybe added to this solution.

The resulting polarizable electrode composition is applied onto acurrent collector to form a polarizable electrode. The method ofapplication is not subject to any particular limitation. Any knownmethod of application, such as one involving the use of a doctor bladeor an air knife, may be suitably employed.

The current collectors used for this purpose may be any positive andnegative electrode current collectors commonly employed in electricaldouble-layer capacitors. The positive electrode current collector ispreferably aluminum foil or aluminum oxide, and the negative electrodecurrent collector is preferably copper foil, nickel foil, or a metalfoil covered on the surface with a copper plating film or a nickelplating film.

The foils making up the respective current collectors may be in any ofvarious shapes, including thin foils, flat sheets, and perforated,stampable sheets. The foil has a thickness of generally about 1 to 200μm. Taking into account such characteristics as the density of theactivated carbon as a portion of the overall electrode and the electrodestrength, a thickness of 8 to 100 μm, and especially 8 to 30 μm, ispreferred.

Alternatively, the polarizable electrodes can be produced by melting andblending the polarizable electrode composition, then extruding the blendas a film.

A conductive material may be added to the above-described activatedcarbon. The conductive material may be any suitable material capable ofconferring electrical conductivity to the activated carbon.Illustrative, non-limiting, examples include carbon black, Ketjenblack,acetylene black, carbon whiskers, carbon fibers, natural graphite,artificial graphite, titanium oxide, ruthenium oxide, and metallicfibers such as those made of aluminum and nickel. Any one orcombinations of two or more thereof may be used. Of these, Ketjenblackand acetylene black, which are both types of carbon black, arepreferred.

The average particle size of the conductive material, though not subjectto any particular limitation, is preferably 10 nm to 10 μm, morepreferably 10 to 100 nm, and even more preferably 20 to 40 nm. It isespecially advantageous for the conductive material to have an averageparticle size which is from 1/5000 to ½, and preferably from 1/1000 to1/10, the average particle size of the activated carbon.

The amount of conductive material added is not subject to any particularlimitation, although an amount of from 0.1 to 20 parts by weight, andpreferably 0.5 to 10 parts by weight, per 100 parts by weight of theactivated carbon is desirable for achieving a good electrostaticcapacitance and imparting electrical conductivity.

The separator may be one that is commonly used in electricaldouble-layer capacitors. Illustrative examples include polyolefinnonwoven fabric, polytetrafluoroethylene porous film, kraft paper, sheetlaid from a blend of rayon fibers and sisal fibers, manila hemp sheet,glass fiber sheet, cellulose-based electrolytic paper, paper made fromrayon fibers, paper made from a blend of cellulose and glass fibers, andcombinations thereof in the form of multilayer sheets.

The electrical double-layer capacitor of the invention can be assembledby stacking, fan-folding or winding an electrical double-layer capacitorassembly composed of the above-described pair of polarizable electrodeswith a separator therebetween. The capacitor assembly is then placedwithin a capacitor housing such as a can or a laminate pack, followingwhich the housing is filled with electrolyte or a polymerelectrolyte-forming composition then mechanically sealed if it is a canor heat-sealed if it is a laminate pack. When a polymerelectrolyte-forming composition is used, this may then be reacted toeffect curing.

The resulting electrical double-layer capacitor of the invention can beoperated at a high capacity and a high current without comprising suchdesirable characteristics as its excellent charge-discharge efficiency,high energy density, high power density and long life. Moreover, it hasa broad service temperature range.

The electrical double-layer capacitors of the invention are highlysuitable for use as a memory backup power supply for cell phones,notebook computers and portable remote terminals, as a power supply forcell phones and portable acoustic devices, as an uninterruptible powersupply for personal computers and other equipment, and as various typesof low-current electrical storage devices such as load leveling powersupplies used in combination with solar power generation and wind powergeneration. Moreover, electrical double-layer capacitors capable ofbeing charged and discharged at a high current are suitable for use ashigh-current electrical storage devices in applications that require alarge current, such as electric cars and electrical power tools.

[Secondary Cells]

The secondary cell according to this invention is a secondary cellhaving a positive electrode which contains a lithium-containing doubleoxide, a negative electrode containing a carbonaceous material capableof lithium ion insertion and extraction or containing metallic lithium,a separator between the positive and negative electrodes, and which isthe above-described nonaqueous electrolyte.

The positive electrode may be one that is produced by coating both thefront and back sides or just one side of a positive electrode currentcollector with a positive electrode binder composition composedprimarily of a binder polymer and a positive electrode active material.

Alternatively, a positive electrode binder composition composedprimarily of a binder polymer and a positive electrode active materialmay be melted and blended, then extruded as a film to form the positiveelectrode.

The binder polymer may be any polymer capable of being used in theapplications of concern here, such as the binder polymers describedabove in connection with electrical double-layer capacitors.

The positive electrode current collector may be made of a suitablematerial such as stainless steel, aluminum, titanium, tantalum ornickel. Of these, aluminum foil or aluminum oxide foil is especiallypreferred both in terms of performance and cost. This current collectormay be used in any of various forms, including foil, expanded metal,sheet, foam, wool, or a three-dimensional structure such as a net.

In this invention, lithium ion-containing chalcogen compounds(lithium-containing double oxides) may be used as the above positiveelectrode active material.

Specific examples of such lithium ion-containing chalcogen compounds(lithium-containing double oxides) include LiCoO₂, LiMnO₂, LiMn₂O₄,LiMo₂O₄, LiV₃O₈, LiNiO₂ and Li_(x)Ni_(y)M_(1−y)O₂ (wherein M is one ormore metal element selected from among cobalt, manganese, titanium,chromium, vanadium, aluminum, tin, lead and zinc; 0.05 ≦x≦1.10; and0.5≦y≦1.0).

In addition to the binder resin and the positive electrode activematerial described above, if necessary, the binder composition for thepositive electrode may include also an electrically conductive material.Illustrative examples of the conductive material include carbon black,Ketjenblack, acetylene black, carbon whiskers, carbon fibers, naturalgraphite and artificial graphite.

The positive electrode binder composition typically includes 1,000 to5,000 parts by weight, and preferably 1,200 to 3,500 parts by weight, ofthe positive electrode active material and 20 to 500 parts by weight,and preferably 50 to 400 parts by weight, of the conductive material per100 parts by weight of the binder polymer.

The negative electrode may be a negative electrode composed of lithiummetal or a negative electrode produced by coating both the front andback sides or just one side of a negative electrode current collectorwith a negative electrode binder composition composed primarily of abinder polymer and a negative electrode active material. The same binderpolymer may be used as in the positive electrode.

Alternatively, the negative electrode binder composition composedprimarily of a binder polymer and a negative electrode active materialmay be melted and blended, then extruded as a film to form a negativeelectrode.

The negative electrode current collector may be made of a suitablematerial such as copper, stainless steel, titanium or nickel. Of these,copper foil or a metal foil whose surface is covered with a copperplating film is especially preferred both in terms of performance andcost. The current collector used may be in any of various forms,including foil, expanded metal, sheet, foam, wool, or athree-dimensional structure such as a net.

The negative electrode active material may be, for example, an alkalimetal, an alkali metal alloy, an oxide, sulfide or nitride of at leastone element selected from among group 8, 9, 10, 11, 12, 13, 14 and 15elements of the periodic table, which oxide, sulfide or nitride iscapable of lithium ion insertion and extraction, or a carbonaceousmaterial capable of reversible lithium ion insertion and extraction.

Examples of suitable alkali metals include lithium, sodium andpotassium. Examples of suitable alkali metal alloys include metalliclithium, Li—Al, Li—Mg, Li—Al—Ni, sodium, Na—Hg and Na—Zn.

Illustrative examples of the oxides of at least one element selectedfrom periodic table group 8 to 15 elements capable of lithium ioninsertion and extraction include tin silicon oxide (SnSiO₃), lithiumbismuth oxide (Li₃BiO₄) and lithium zinc oxide (Li₂ZnO₂).

Illustrative examples of the sulfides include lithium iron sulfidesLi_(x)FeS₂ (wherein 0≦x≦3) and lithium copper sulfides Li_(x)CuS(wherein 0≦x≦3).

Illustrative examples of the nitrides include lithium-containingtransition metal nitrides, and specifically Li_(x)M_(y)N (wherein M iscobalt, nickel or copper; 0≦x≦3; and 0≦y≦0.5) and lithium iron nitride(Li₃FeN₄).

Examples of carbonaceous materials which are capable of reversiblelithium ion insertion and extraction include lo graphite, carbon black,coke, glassy carbon, carbon fibers, and sintered bodies obtained fromany of these.

If necessary, a conductive material may be added to the negativeelectrode binder composition as well. Examples of suitable conductivematerials include the same materials as those mentioned above inconnection with the positive electrode binder.

The negative electrode binder composition typically includes 500 to1,700 parts by weight, preferably 700 to 1,300 parts by weight, of thenegative electrode active material and 0 to 70 parts by weight,preferably 0 to 40 parts by weight, of the conductive material per 100parts by weight of the binder polymer.

The above-described negative electrode binder compositions and positiveelectrode binder compositions generally are used in the form of a pasteafter the addition of a dispersing medium. Suitable dispersing mediainclude polar solvents such as N-methyl-2-pyrrolidone (NMP),dimethylformamide, dimethylacetamide and dimethylsulfamide. Thedispersing medium is typically added in an amount of about 30 to 300parts by weight per 100 parts by weight of the positive electrode ornegative electrode binder composition.

No particular limitation is imposed on the method of shaping thepositive and negative electrodes as thin films, although it ispreferable to apply the composition by a suitable means such as rollercoating with an applicator roll, screen coating, doctor blade coating,spin coating or bar coating so as to form an active material layerhaving a uniform thickness when dry of 10 to 200 μm, and especially 50to 150 μm.

Illustrative, non-limiting, examples of the separator between thepositive and negative electrodes include polyethylene nonwoven fabric,polypropylene nonwoven fabric, polyester nonwoven fabric,polytetrafluoroethylene porous film, kraft paper, sheet laid from ablend of rayon fibers and sisal fibers, manila hemp sheet, glass fibersheet, cellulose-based electrolytic paper, paper made from rayon fibers,paper made from a blend of cellulose and glass fibers, and combinationsthereof in the form of multilayer sheets.

The secondary cell of the invention is assembled by stacking,fan-folding, winding or forming into a laminated or coin-like shape acell assembly composed of the separator disposed between the positiveand negative electrodes, and placing the cell assembly within a batteryhousing such as a battery can or a laminate pack. The battery housing ismechanically sealed if it is a can or heat-sealed if it is a laminatepack. In constructing the cell, the separator is disposed between thepositive electrode and the negative electrode, the resulting cellassembly is placed within the battery housing, then the cell assembly isfilled with nonaqueous electrolyte. If a compound having reactive doublebonds is used as the nonaqueous electrolyte, the electrolyte compositionis filled into the cell assembly so that it fully penetrates the gapbetween the electrodes and the gaps between the separator and theelectrodes, then is reacted to effect curing.

The resulting nonaqueous electrolyte secondary cell of the invention canbe operated at a high capacity and a high current without compromisingsuch desirable characteristics as its excellent charge-dischargeefficiency, high energy density, high output density and long life.Moreover, the cell has a broad service temperature range.

The nonaqueous electrolyte secondary cell of the invention lends itselfwell to use in a variety of applications, including main power suppliesand memory backup power supplies for portable electronic equipment suchas video cameras, notebook computers, cell phones and PHS (“personalhandyphone system”) devices, uninterruptible power supplies forequipment such as personal computers, in electric cars and hybrid cars,and together with solar cells as energy storage systems for solar powergeneration.

EXAMPLE

The following synthesis examples, examples of the invention andcomparative examples are provided to illustrate the invention and do notin any way limit the invention.

Synthesis Example 1

Synthesis of Compound (6)

A solution prepared by mixing together 100 mL of diethylamine (KantoChemical Co., Inc.) and 85 mL of 2-methoxyethyl chloride (KantoChemical) was placed in an autoclave and reacted at 100° C. for 24hours. The internal pressure during the reaction was 0.127 MPa (1.3kgf/cm²). This yielded a mixture of deposited crystals and reactionsolution to which was added, following the 24 hours of reaction, 200 mLof an aqueous solution containing 56 g of dissolved potassium hydroxide(Katayama Chemical Industries Co., Ltd.). Each of the two dividedorganic phases that formed as a result was separated off with aseparatory funnel and subjected twice to extraction with 100 mL ofmethylene chloride (Wako Pure Chemical Industries, Ltd.). The separatedorganic phases were then combined and washed with a saturated salinesolution, following which potassium carbonate (Wako Pure ChemicalIndustries) was added to remove water and vacuum filtration was carriedout. The solvent in the resulting organic phase was distilled off in arotary evaporator, after which the residue was subjected tonormal-pressure distillation, yielding 18.9 g of a fraction having aboiling point close to 135° C. This was confirmed from a ¹H-NMR spectrumto be 2-methoxyethyldiethylamine.

Next, 8.24 g of the 2-methoxyethyldiethylamine was dissolved in 10 mL oftetrahydrofuran (Wako Pure Chemical Industries), then 4.0 mL of methyliodide (Wako Pure Chemical Industries) was added under ice cooling.After 30 minutes, the mixture was removed from the ice bath and stirredovernight at room temperature. The solvent in this reaction solution wassubsequently driven off by vacuum distillation, and the resulting solidswere recrystallized from an ethanol (Wako Pure ChemicalIndustries)—tetrahydrofuran system, yielding 16 g of2-methoxyethyldiethylmethylammonium iodide.

Next, 15.0 g of the 2-methoxyethyldiethylmethyl ammonium iodide wasdissolved in 100 mL of distilled water, after which 6.37 g of silveroxide (Kanto Chemical) was added and the mixture was stirred for 3hours. This reaction mixture was vacuum filtered to remove precipitates,following which 42% tetrafluoroboric acid (Kanto Chemical) was added alittle at a time under stirring until the reaction solution reached a pHof about 5 to 6. This reaction solution was then freeze-dried and waterwas thoroughly driven off with a vacuum pump, yielding 12.39 g ofcompound (6) which was liquid at room temperature (25° C.).

Synthesis Example 2

Synthesis of Compound (11)

2-Methoxyethyldiethylmethylammonium iodide (10.0 g) obtained by the samemethod as in Synthesis Example 1 was dissolved in 50 mL of acetonitrile(Kanto Chemical), following which 9.5 g of lithiumbis(trifluoromethanesulfonyl)imide (produced by Kishida Chemical Co.,Ltd.) was added and completely dissolved therein, and the resultingsolution was stirred for 15 minutes.

The acetonitrile was removed by vacuum distillation, following whichwater was added to the residue, causing the organic phase to divide intotwo. The organic phases were then separated off and washed five timeswith water to remove impurities.

The washed organic phases were subsequently placed under reducedpressure with a vacuum pump and the water was thoroughly driven off,yielding 6.8 g of compound (11), which was liquid at room temperature.

Synthesis Example 3

Synthesis of Thermoplastic Polyurethane Resin

A reactor equipped with a stirrer, a thermometer and a condensing tubewas charged with 60.20 parts by weight of a preheated and dehydratedpolyethylene glycol 4000 (PEG 4000-S, made by Sanyo Chemical Industries,Ltd.) and 7.84 parts by weight of 4,4′-diphenylmethane diisocyanate. Thereactor contents were stirred and mixed for 2 hours at 120° C. under astream of nitrogen, following which 1.86 parts by weight of1,4-butanediol was added to the mixture and the reaction was similarlyeffected at 120° C. under a stream of nitrogen. When the reactionreached the point where the reaction product became rubbery, it wasstopped. The reaction product was then removed from the reactor andheated at 100° C. for 12 hours. Once the isocyanate peak was confirmedto have disappeared from the infrared absorption spectrum, heating wasstopped, yielding a solid polyurethane resin.

The resulting polyurethane resin had a weight-average molecular weight(Mw) of 1.05×10⁵.

Synthesis Example 4

Synthesis of Cellulose Derivative

Eight grams of hydroxypropyl cellulose (molar substitution, 4.65;product of Nippon Soda Co., Ltd.) was suspended in 400 mL ofacrylonitrile, following which 1 mL of 4 wt % aqueous sodium hydroxidewas added and the mixture was stirred 4 hours at 30° C. The reactionmixture was then neutralized with acetic acid and poured into a largeamount of methanol, giving cyanoethylated hydroxypropyl cellulose.

To remove impurities, the cyanoethylated hydroxypropyl cellulose wasdissolved in acetone, following which the solution was placed in adialysis membrane tube and purified by dialysis using ion-exchangedwater. The cyanoethylated hydroxypropyl cellulose which settled outduring dialysis was collected and dried.

Elemental analysis of the resulting cyanoethylated hydroxypropylcellulose indicated a nitrogen content of 7.3 wt %. Based on this value,the proportion of the hydroxyl groups on the hydroxypropyl cellulosethat were capped with cyanoethyl groups was 94%.

Synthesis Example 5

Synthesis of Oxyalkylene-Branched Polyvinyl Alcohol Derivative

A reactor equipped with a stirring element was charged with 10 parts byweight of polyvinyl alcohol (average degree of polymerization, 500;vinyl alcohol fraction, ≧98%) and 70 parts by weight of acetone. Asolution of 1.81 parts by weight of sodium hydroxide in 2.5 parts byweight of water was gradually added under stirring, after which stirringwas continued for one hour at room temperature. To this solution wasgradually added, over a period of 3 hours, a solution of 67 parts byweight of glycidol in 100 parts by weight of acetone. The resultingmixture was stirred for 8 hours at 50° C. to effect the reaction.Following reaction completion, stirring was stopped, whereupon thepolymer precipitated from the mixture. The precipitate was collected,dissolved in 400 parts by weight of water, and neutralized with aceticacid. The neutralized polymer was purified by dialysis, and theresulting solution was freeze-dried, giving 22.50 parts by weight ofdihydroxypropylated polyvinyl alcohol.

Given that polyvinyl alcohol has a unit molecular weight of 44 andglycidol has a unit molecular weight of 74, a polyvinyl alcoholderivative obtained by the addition of n units of glycidol (molarsubstitution) would have a unit molecular weight of 44+74n. Such apolyvinyl alcohol derivative would thus have an average molarsubstitution (MS), as calculated based on this unit molecular weight,the weight of the polyvinyl alcohol charged, and the weight of theproduct obtained, of n=0.74.

Based on a ¹³C-NMR spectrum (DEPT spectrum measured using a VarianVXR-300 NMR spectrometer, with D₂O as the solvent) of this product, theaverage molar substitution (average MS) determined by comparing the C*carbon signal intensity (A) for —C*H₂—C(OH)H— units from the unreactedpolyvinyl alcohol with the signal intensity (C) for other carbons was0.95.

In addition, the fraction of unreacted —(CH₂—C(OH)H)— units wasdetermined by comparing the signal intensities (A) and (C). Thisunreacted fraction a was 0.57.

Accordingly, of the diydroxypropyl groups (DHP) which formed as a resultof glycidol addition, the fraction b that reacted was 1−a, or 0.43, andthe average length (L) of the DHP chain was L=MS/b=2.21.

Three parts by weight of the resulting polyvinyl alcohol was mixed with20 parts by weight of dioxane and 14 parts by weight of acrylonitrile.To this mixed solution was added a solution of 0.16 part by weight ofsodium hydroxide in 1 part by weight of water, and stirring was carriedout for 10 hours at 25° C. The resulting mixture was neutralized usingan ion-exchange resin (Amberlite IRC-76, produced by OrganoCorporation). The ion-exchange resin was separated off by filtration,after which 50 parts by weight of acetone was added to the solution andthe insolubles were filtered off. The resulting acetone solution wasplaced in dialysis membrane tubing and dialyzed with running water. Thepolymer which precipitated within the dialysis membrane tubing wascollected and re-dissolved in acetone. The resulting solution wasfiltered, following which the acetone was evaporated off, giving acyanoethylated polyvinyl alcohol derivative.

The infrared absorption spectrum of this polymer derivative showed nohydroxyl group absorption, confirming that all the hydroxyl groups werecapped with cyanoethyl groups (capping ratio, 100%).

Synthesis Example 6

Synthesis of Cyano-Substituted Monovalent Hydrocarbon Group-BearingPolyvinyl Alcohol Derivative

A reaction vessel equipped with a stirring element was charged with 3parts by weight of polyvinyl alcohol (average degree of polymerization,500; vinyl alcohol fraction, ≧98%), 20 parts by weight of 1,4-dioxaneand 14 parts by weight of acrylonitrile. To this mixture was graduallyadded a solution of 0.16 part by weight of sodium hydroxide in 1 part byweight of water, and stirring was carried out for 10 hours at 25° C.

The resulting mixture was neutralized using an ion-exchange resin(Amberlite IRC-76, produced by Organo Corporation). The ion-exchangeresin was then separated off by filtration, after which 50 parts byweight of acetone was added to the solution and the insolubles werefiltered off. The resulting acetone solution was placed in dialysismembrane tubing and dialyzed with running water. The polymer whichprecipitated within the dialysis membrane tubing was collected andre-dissolved in acetone. The resulting solution was filtered, followingwhich the acetone was evaporated off, giving a cyanoethylated polyvinylalcohol derivative.

The infrared absorption spectrum of this polymer derivative showed nohydroxyl group absorption, confirming that all the hydroxyl groups werecapped with cyanoethyl groups (capping ratio, 100%).

Example 1 Nonaqueous Electrolyte

[Production of Polyurethane Resin Film]

Five parts by weight of the polyurethane resin obtained in SynthesisExample 3 and 95 parts by weight of N-methyl-2-pyrrolidone were stirredand mixed, giving a polyurethane resin solution. The resultingpolyurethane resin solution was applied by means of a doctor blade so asto give a film thickness when dry of 30 μm, then dried in vacuo at 120°C. for 2 hours to give a polyurethane resin film.

[Measurement of Relative Permittivity]

The polyurethane resin film obtained as described above was cut to asize of 4×4 cm, and the relative permittivity of the film at 25° C. anda frequency of 1 MHz was measured using an RF impedance/materialanalyzer (model 4291B, manufactured by Agilent Technologies, Inc.). Therelative permittivity was 16.2.

[Production of Polymer Electrolyte Membrane]

The polyurethane resin film produced as described above was immersed for24 hours in the ionic liquid prepared in Synthesis Example 1 and therebyimpregnated with liquid electrolyte, giving a polymer electrolytemembrane.

[Measurement of Ionic Conductivity]

The resulting polymer electrolyte membrane was placed between two sheetsof copper, and the ionic conductivity at 25° C. was measured by the ACimpedance method. The ionic conductivity was 2.2×10⁻³ S/cm.

[Measurement of Potential Window]

The potential window of the polymer electrolyte membrane produced asdescribed above was measured by potensiostat and function generatorusing platinum electrodes as the working electrode and the counterelectrode, and using a silver/silver chloride electrode as the referenceelectrode. The polymer electrolyte membrane was thus found to have apotential window with respect to a silver/silver chloride electrode of−3.0 V to +3.0 V.

Comparative Example 1

[Production of Polyvinyl Chloride Resin Film]

Five parts by weight of polyvinyl chloride resin and 95 parts by weightof tetrahydrofuran were stirred and mixed, giving a polyvinyl chlorideresin solution. The resulting polyvinyl chloride resin solution wasapplied by means of a doctor blade so as to give a film thickness whendry of 30 μm, then dried in vacuo at 120° C. for 2 hours to give apolyvinyl chloride resin film.

[Measurement of Relative Permittivity]

The polyvinyl chloride resin film obtained as described above was cut toa size of 4×4 cm, and the relative permittivity of the film was measuredin the same way as in Example 1. The relative permittivity was 3.1.

[Production of Polymer Electrolyte Membrane]

The polyvinyl chloride resin film produced as described above wasimmersed for 24 hours in the ionic liquid prepared in Synthesis Example1 and thereby impregnated with liquid electrolyte, giving a polymerelectrolyte membrane.

[Measurement of Ionic Conductivity]

The ionic conductivity at 25° C. of the resulting polymer electrolytemembrane was measured in the same way as in Example 1, and found to be9.6×10⁻⁴ S/cm.

Comparative Example 2

[Production of Polymer Electrolyte Membrane]

The polyurethane resin film obtained in Example 1 was immersed for 24hours in 1-ethyl-3-methylimidazolium tetrafluoroborate (Aldrich ChemicalCo., Ltd.) and thereby impregnated with a liquid electrolyte, giving apolymer electrolyte membrane.

[Measurement of Potential Window]

The potential window of the polymer electrolyte membrane produced asdescribed above was measured in the same way as in Example 1, and wasfound to have a potential window with respect to a silver/silverchloride electrode of −1.8 V to +2.7 V.

From the above, it is apparent that the polymer electrolyte membrane ofExample 1 composed of the ionic liquid of Synthesis Example 1 and apolyurethane resin as the conductive polymer had a better ionicconductivity than the polymer electrolyte membrane of ComparativeExample 1 composed of the ionic liquid of Synthesis Example 1 andpolyvinyl chloride. Moreover, it is apparent that a polymer electrolytemembrane in which an ionic liquid according to the invention was usedhad a broader potential window than a polymer electrolyte membrane inwhich an imidazolium-based ionic liquid was used.

Example 2 Electrical Double-Layer Capacitor 1

[Production of Polarizable Electrodes]

A polarizable electrode composition in the form of a paste was preparedby stirring and mixing together 85 parts by weight of activated carbon(MSP20, produced by Kansai Coke and Chemicals Co., Ltd.), 10 parts byweight of acetylene black, 50 parts by weight of a solution of 10 partsby weight polyvinylidene fluoride dissolved in 90 parts by weightN-methyl-2-pyrrolidone, and 165 parts by weight ofN-methyl-2-pyrrolidone. This polarizable electrode composition wascoated onto aluminum oxide foil with a doctor blade, then dried at 80°C. for 2 hours and roll-pressed to an electrode thickness of 30 μm,thereby giving a polarizable electrode.

[Fabrication of Electrical Double-Layer Capacitor]

Two 12 mm diameter disks were cut from the polarizable electrodeproduced as described above. The disks were impregnated with the ionicliquid prepared in Synthesis Example 1 by 30 minutes of immersion in theliquid under a vacuum. In addition, a 13 mm diameter disk was cut fromthe polyurethane resin film produced in Example 1, and impregnated withliquid electrolyte by 24 hours of immersion in the ionic liquid preparedin Synthesis Example 1. The two polarizable electrodes impregnated withthis liquid electrolyte were stacked together, with the ionicliquid-impregnated polyurethane resin film therebetween. The resultingassembly was sealed in an outer case, thereby giving an electricaldouble-layer capacitor.

[Charge/Discharge Test]

The resulting electrical double-layer capacitor was subjected toconstant current charge-discharge at a cutoff voltage during charging of2.5 V, an end-of-discharge voltage of 0 V and a current density of 1.5mA/cm². The electrostatic capacitance per polarizable electrode, ascomputed from the integrated electrical energy during discharge, was33.4 F/g.

Example 3 Electrical Double-Layer Capacitor 2

[Production of Cellulose Derivative Film]

Five parts by weight of the cellulose derivative obtained in SynthesisExample 4 and 95 parts by weight of propylene carbonate were stirred andmixed, giving a cellulose derivative solution. The resulting cellulosederivative solution was applied by means of a doctor blade so as to givea film thickness when dry of 30 μm, then dried in vacuo at 120° C. for 2hours to form a cellulose derivative film.

[Fabrication of Electrical Double-Layer Capacitor]

Aside from using the cellulose derivative film prepared above instead ofthe polyurethane resin film used in Example 1, an electricaldouble-layer capacitor was fabricated in the same way as in Example 1.

[Charge/Discharge Test]

The resulting electrical double-layer capacitor was subjected to acharge/discharge test under the same conditions as in Example 1. Theelectrostatic capacitance per polarizable electrode was 32.1 F/g.

Example 4 Electrical Double-Layer Capacitor 3

[Production of Oxyalkylene-Branched Polyvinyl Alcohol Derivative Film]

Five parts by weight of the oxyalkylene-branched polyvinyl alcoholderivative obtained in Synthesis Example 5 and 95 parts by weight ofpropylene carbonate were stirred and mixed, giving anoxyalkylene-branched polyvinyl alcohol derivative solution. Theresulting oxyalkylene-branched polyvinyl alcohol derivative solution wasapplied by means of a doctor blade so as to give a film thickness whendry of 30 μm, then dried in vacuo at 120° C. for 2 hours to form anoxyalkylene-branched polyvinyl alcohol derivative film.

[Fabrication of Electrical Double-Layer Capacitor]

Aside from using the oxyalkylene-branched polyvinyl alcohol derivativefilm prepared above instead of the polyurethane resin film used inExample 1, an electrical double-layer capacitor was fabricated in thesame way as in Example 1.

[Charge/Discharge Test]

The resulting electrical double-layer capacitor was subjected to acharge/discharge test under the same conditions as in Example 1. Theelectrostatic capacitance per polarizable electrode was 33.0 F/g.

Example 5 Electrical Double-Layer Capacitor 4

[Production of Cyano-Substituted Monovalent Hydrocarbon Group-BearingPolyvinyl Alcohol Derivative Film]

Five parts by weight of the cyano-substituted monovalent hydrocarbongroup-bearing polyvinyl alcohol derivative obtained in Synthesis Example6 and 95 parts by weight of propylene carbonate were stirred and mixed,giving a cyano-substituted monovalent hydrocarbon group-bearingpolyvinyl alcohol derivative solution. The resulting cyano-substitutedmonovalent hydrocarbon group-bearing polyvinyl alcohol derivativesolution was applied by means of a doctor blade so as to give a filmthickness when dry of 30 μm, then dried in vacuo at 120° C. for 2 hoursto form a cyano-substituted monovalent hydrocarbon group-bearingpolyvinyl alcohol derivative film.

[Fabrication of Electrical Double-Layer Capacitor]

Aside from using the cyano-substituted monovalent hydrocarbongroup-bearing polyvinyl alcohol derivative film prepared above insteadof the polyurethane resin film used in Example 1, an electricaldouble-layer capacitor was fabricated in the same way as in Example 1.

[Charge/Discharge Test]

The resulting electrical double-layer capacitor was subjected to acharge/discharge test under the same conditions as in Example 1. Theelectrostatic capacitance per polarizable electrode was 32.4 F/g.

Example 6 Electrical Double-Layer Capacitor 6

[Preparation of Electrolyte-Forming Composition Solution]

The following dehydration-treated components were mixed in the indicatedamounts: 100 parts by weight of polyethylene glycol dimethacrylate(number of oxirene units, 9), 70.15 parts by weight ofmethoxypolyethylene glycol monomethacrylate (number of oxirene units,2), 8.41 parts by weight of trimethylolpropane trimethacrylate, and178.56 parts by weight of the cyano-substituted monovalent hydrocarbongroup-bearing polyvinyl alcohol derivative obtained in Synthesis Example6. Next, 85 parts by weight of the ionic liquid prepared in SynthesisExample 1 and 0.5 part by weight of azobisisobutyronitrile were added to14.5 parts by weight of this mixed composition, thereby giving anelectrolyte-forming composition.

[Fabrication of Electrical Double-Layer Capacitor]

Two polarizable electrodes fabricated in the same way as in Example 1were cut to a diameter of 12 mm, and a cellulose separator (TF 40-35,made by Nippon Kodoshi Corporation) was cut to a diameter of 13 mm.These were impregnated with the electrolyte-forming composition solutionprepared above by 30 minutes of immersion in the solution under avacuum. The two polarizable electrodes impregnated with theelectrolyte-forming composition solution were stacked together, with theelectrolyte-forming composition solution-impregnated separatortherebetween. The resulting assembly was sealed in an outer case andsubsequently heated at 55° C. for 2 hours and at 80° C. for 0.5 hour toeffect gelation, thereby giving an electrical double-layer capacitor.

[Charge/Discharge Test]

The resulting electrical double-layer capacitor was subjected to acharge/discharge test under the same conditions as in Example 1. Theelectrostatic capacitance per polarizable electrode was 31.5 F/g.

Example 7 Secondary Cell 1

[Production of Positive Electrode]

Ninety-two parts by weight of LiCoO₂ as the positive electrode activematerial, 3 parts by weight of Ket enblack as the conductive material,50 parts by weight of a solution of 10 parts by weight of polyvinylidenefluoride in 90 parts by weight of N-methyl-2-pyrrolidone, and 20 partsby weight of N-methyl-2-pyrrolidone were stirred and mixed together,giving a paste-like positive electrode composition. This positiveelectrode composition was applied onto aluminum foil with a doctorblade, then dried at 80° C. for 2 hours and roll-pressed to an electrodethickness of 30 μm, thereby forming a positive electrode.

[Production of Negative Electrode]

Ninety-two parts by weight of mesophase carbon microbeads (MCMB 6-28,made by Osaka Gas Chemicals Co., Ltd.) as the negative electrode activematerial, 80 parts by weight of a solution of 10 parts by weight ofpolyvinylidene fluoride in 90 parts by weight of N-methyl-2-pyrrolidone,and 40 parts by weight of N-methyl-2-pyrrolidone were stirred and mixedtogether, giving a paste-like negative electrode composition. Thisnegative electrode composition was applied onto copper foil with adoctor blade, then dried at 80° C. for 2 hours and roll-pressed to anelectrode thickness of 30 μm, thereby forming a negative electrode.

[Preparation of Electrolyte Solution]

An electrolyte solution was prepared by dissolving 4 parts by weight oflithium bis(trifluoromethane)imide in 96 parts by weight of the ionicliquid obtained in Synthesis Example 2.

[Fabrication of Secondary Cell]

The positive electrode and negative electrode obtained as describedabove were cut to respective diameters of 11 mm and 12 mm, then wereimpregnated with the electrolyte solution prepared above by 30 minutesof immersion in the solution under a vacuum. In addition, thepolyurethane resin film produced in Example 1 was cut to a diameter of13 mm and immersed for 24 hours in the electrolyte solution prepared asdescribed above so as to impregnate it with the solution. Theelectrolyte solution-impregnated positive and negative electrodes werestacked together, with the electrolyte solution-impregnated polyurethaneresin film therebetween, and the resulting assembly was sealed in anouter case to give a secondary cell.

[Charge/Discharge Test]

The secondary cell produced as described above was subjected to acharge/discharge test at a charge voltage of 4.2 mV, a discharge voltageof 2.7 V, and under a constant current at a current density of 0.03mA/cm². The cell was found to have a capacity of 0.705 mAh and acharge-discharge efficiency in the first cycle of 73.8%.

Example 8 Secondary Cell 2

[Fabrication of Secondary Cell]

Aside from using the cellulose derivative film produced in Example 3instead of a polyurethane resin film, a secondary cell was produced inthe same way as in Example 7.

[Charge/Discharge Test]

The resulting secondary cell was subjected to a charge/discharge testunder the same conditions as in Example 7. The cell capacity was 0.698mAh and the charge-discharge efficiency in the first cycle was 73.2%.

Example 9 Secondary Cell 3

[Fabrication of Secondary Cell]

Aside from using the oxyalkylene-branched polyvinyl alcohol derivativefilm produced in Example 4 instead of a polyurethane resin film, asecondary cell was produced in the same way as in Example 7.

[Charge/Discharge Test]

The resulting secondary cell was subjected to a charge/discharge testunder the same conditions as in Example 7. The cell capacity was 0.703mAh and the charge-discharge efficiency in the first cycle was 73.6%.

Example 10 Secondary Cell 4

[Fabrication of Secondary Cell]

Aside from using the cyano-substituted monovalent hydrocarbon-bearingpolyvinyl alcohol derivative film produced in Example 6 instead of apolyurethane resin film, a secondary cell was produced in the same wayas in Example 7.

[Charge/Discharge Test]

The resulting secondary cell was subjected to a charge/discharge testunder the same conditions as in Example 7. The cell capacity was 0.700mAh and the charge-discharge efficiency in the first cycle was 73.0%.

Example 11 Secondary Cell 6

[Preparation of Electrolyte-Forming Composition Solution]

The following dehydration-treated components were mixed in the indicatedamounts: 100 parts by weight of polyethylene glycol dimethacrylate(number of oxirene units, 9), 70.15 parts by weight ofmethoxypolyethylene glycol monomethacrylate (number of oxirene units,2), 8.41 parts by weight of trimethyloipropane trimethacrylate, and178.56 parts by weight of the cyano-substituted monovalent hydrocarbongroup-bearing polyvinyl alcohol derivative obtained in Synthesis Example6. Next, 85 parts by weight of the electrolyte solution prepared inExample 7 and 0.5 part by weight of azobisisobutyronitrile were added to14.5 parts by weight of this mixed composition, giving an electrolytecomposition.

[Fabrication of Secondary Cell]

A positive electrode and a negative electrode obtained in the same wayas in Example 7 were cut to respective diameters of 11 mm and 12 mm, anda cellulose separator (TF 40-30, made by Nippon Kodoshi Corporation) wascut to a diameter of 13 mm. All three were impregnated with theelectrolyte-forming composition solution prepared above by 30 minutes ofimmersion in the solution under a vacuum. The positive electrode andnegative electrode impregnated with the electrolyte-forming compositionsolution were stacked together, with the electrolyte-forming compositionsolution-impregnated separator therebetween. The resulting assembly wassealed within an outer case, then heated at 55° C. for 2 hours and at80° C. for 0.5 hour to induce gelation, thereby giving a secondary cell.

[Charge/Discharge Test]

The resulting secondary cell was subjected to a charge/discharge testunder the same conditions as in Example 7. The cell had a capacity of0.692 mAh and a charge-discharge efficiency in the first cycle of 73.1%.

Example 12 Secondary Cell 7

[Fabrication of Electrolyte Solution]

An electrolyte solution was prepared by dissolving 4 parts by weight oflithium bis(trifluoromethanesulfonyl)imide in 96 parts by weight of theionic liquid obtained in Synthesis Example 2, then adding 10 parts byweight of vinylene carbonate.

[Production of Secondary Cell]

Aside from using the electrolyte solution prepared as described above, asecondary cell was produced in the same way as in Example 7.

[Charge/Discharge Test]

The resulting secondary cell was subjected to a charge/discharge testunder the same conditions as in Example 7. The cell capacity was 0.708mAh and the charge-discharge efficiency in the first cycle was 75.5%.

Example 13 Secondary Cell 8

[Fabrication of Secondary Cell]

The positive electrode produced in Example 7 was cut to a diameter of 11mm, then was impregnated with the electrolyte solution prepared inExample 7 by 30 minutes of immersion in the solution under a vacuum. Inaddition, the polyurethane resin film produced in Example 1 was cut to adiameter of 13 mm and impregnated with the electrolyte solution preparedin Example 7 by 24 hours of immersion therein. This electrolytesolution-impregnated positive electrode and a 12 mm diameter stampedlithium metal disk were stacked together, with an electrolytesolution-impregnated polyurethane resin film in between, and theresulting assembly was sealed in an outer case to form a secondary cell.

[Charge/Discharge Test]

The resulting secondary cell was subjected to a charge/discharge testunder the same conditions as in Example 7. The cell capacity was 0.695mAh and the charge-discharge efficiency in the first cycle was 72.7%.

Example 14 Secondary Cell 9

[Fabrication of Secondary Cell]

A positive electrode produced in the same way as in Example 7 was cut toa diameter of 11 mm, and a cellulose separator (TF 40-30, made by NipponKodoshi Corporation) was cut to a diameter of 13 mm. These were bothimpregnated with an electrolyte-forming composition solution prepared inthe same way as in Example 11 by 30 minutes of immersion in the solutionunder a vacuum. This electrolyte-forming compositionsolution-impregnated positive electrode and a 12 mm diameter stampedlithium metal disk were stacked together, with an electrolyte-formingcomposition solution-impregnated separator therebetween. The resultingassembly was sealed within an outer case, then heated at 55° C. for 2hours and at 80° C. for 0.5 hour to induce gelation, thereby giving asecondary cell.

[Charge/Discharge Test]

The resulting secondary cell was subjected to a charge/discharge testunder the same conditions as in Example 7. The cell capacity was 0.688mAh and the charge-discharge efficiency in the first cycle was 72.2%.

1. A nonaqueous electrolyte characterized by containing: an ionic liquidwhich has general formula (1) below and is liquid at not higher than 50°C.

wherein R¹ to R⁴ are each independently an alkyl group of 1 to 5 carbonsor an alkoxyalkyl group of the formula R′—O—(CH₂)_(n)— (R′ being methylor ethyl, and the letter n being an integer from 1 to 4), and any twofrom among R¹, R², R³ and R⁴ may together form a ring, with the provisothat at least one of R¹ to R⁴ is an alkoxyalkyl group of the aboveformula, X is a nitrogen atom or a phosphorus atom, and Y is amonovalent anion; and an ion-conductive polymer having a relativepermittivity at 25° C. and 1 MHz of 5 to
 50. 2. A nonaqueous electrolytewhich is characterized in that it is obtained by curing a compositioncontaining: an ionic liquid which has general formula (1) below and isliquid at not higher than 50° C.

wherein R¹ to are each independently an alkyl group of 1 to 5 carbons oran alkoxyallcyl group of the formula R′—O—(CH₂)_(n)— (R¹ being methyl orethyl, and the letter n being an integer from 1 to 4), and any two fromamong R¹, R², R³ and R⁴ may together form a ring, with the proviso thatat least one of R¹ to R⁴ is an alkoxyalkyl group of the above formula, Xis a nitrogen atom or a phosphorus atom, and Y is a monovalent anion; acompound having a reactive double bond on the molecule; and anion-conductive polymer.
 3. The nonaqueous electrolyte of claim 2, whichis characterized in that the ion-conductive polymer has a relativepermittivity at 25° C. and 1 MHz of 5 to
 50. 4. The nonaqueouselectrolyte of claim 1 or 2 which is characterized by containing alithium salt.
 5. The nonaqueous electrolyte of claim 4 which ischaracterized in that the lithium salt is LiBF₄, LiPF₆, Li(CF₃SO₂)₂N,LiCF₃SO₃ or LiCF₃CO₂.
 6. The nonaqueous electrolyte of claim 1 or 2,which is characterized in that the ion-conductive polymer is anoncrystalline polymer.
 7. The nonaqueous electrolyte of claim 1 or 2,which is characterized in that the ion-conductive polymer is athermoplastic polyurethane resin.
 8. The nonaqueous electrolyte of claim1 or 2, which is characterized in that the ion-conductive polymer is ahydroxyalkyl polysaccharide or a hydroxyalkyl polysaccharide derivative.9. The nonaqueous electrolyte of claim 1 or 2, which is characterized inthat the ion-conductive polymer is a polymeric compound having anaverage degree of polymerization of at least 20 and containing polyvinylalcohol groups of general formula (2) below

wherein n is a number from 20 to 10,000, some or all of the hydroxylgroups on the polyvinyl alcohol units being substituted withoxyalkylene-bearing units having an average molar substitution of atleast 0.3.
 10. The nonaqueous electrolyte of claim 1 or 2, which ischaracterized in that the ion-conductive polymer is a polymeric compoundhaving an average degree of polymerization of at least 20 and containingpolyvinyl alcohol units of general formula (2) below

wherein n is a number from 20 to 10,000, some or all of the hydroxylgroups on the polyvinyl alcohol units being substituted withcyano-substituted monovalent hydrocarbon groups.
 11. The nonaqueouselectrolyte of claim 1 or 2, which is characterized in that theion-conductive polymer is a polymeric compound having units of formula(3) and units of formula (4)

wherein at least 10% of the end groups on the molecular chain are cappedwith one or more groups selected from among halogen atoms, substitutedor unsubstituted monovalent hydrocarbon groups, R⁵CO— groups (R⁵ being asubstituted or unsubstituted monovalent hydrocarbon group), R⁵ ₃Si—groups (R⁵ being the same as above), amino groups, alkylamino groups,H(OR⁶)_(m)— groups (R⁶ being an alkylene group of 2 to 4 carbons, and mbeing an integer from 1 to 100) and phosphorus atom-containing groups.12. The nonaqueous electrolyte of claim 1 or 2, which is characterizedin that the ionic liquid is liquid at not higher than 25° C.
 13. Thenonaqueous electrolyte of claim 1 or 2, which is characterized in that Xis a nitrogen atom, R′ is methyl, and n is
 2. 14. The nonaqueouselectrolyte of claim 1 or 2, which is characterized in that the ionicliquid has general formula (5) below

wherein R′ is methyl or ethyl, X is a nitrogen atom or a phosphorusatom, Y is a monovalent anion, Me stands for methyl and Et stands forethyl.
 15. The nonaqueous electrolyte of claim 1 or 2, which ischaracterized in that Y is BF₄ ⁻, PF₆ ⁻, (CF₃SO₂)₂N⁻, CF₃SO₃ ⁻ or CF₃CO₂⁻.
 16. An electrical double-layer capacitor comprising a pair ofpolarizable electrodes, a separator between the polarizable electrodesand a nonaqueous electrolyte, which electrical double-layer capacitor ischaracterized in that the nonaqueous electrolyte is a nonaqueouselectrolyte according to claim 1 or
 2. 17. A nonaqueous electrolytesecondary cell comprising a positive electrode which contains alithium-containing double oxide, a negative electrode which contains acarbonaceous material capable of lithium ion insertion and extraction orcontains metallic lithium, a separator between the positive and negativeelectrodes, and a nonaqueous electrolyte; which nonaqueous secondarycell is characterized in that the nonaqueous electrolyte is a nonaqueouselectrolyte according to claim 1 or 2.